Photo-catalytic hydrogen generation from water is one of the most favorable ways to generate clean energy. Water splitting using semiconductor powder catalysts is a promising and preferred process because of the simplicity and ease of handling.
Semiconductor oxides like TiO2 and ZnO are widely used in photocatalysis due to unique electronic structures facilitating the formation of electron-hole pairs, on irradiation with light of appropriate energy, which can be utilized in electron transfer processes. However, efficient utilization of these charge carriers is possible only if charge recombination is avoided.
Recombination occurs when photo generated electrons reoccupy the valence band and depends on the positions of valence and conduction bands, or electron and hole conduction pathways within the oxide lattice. On irradiation with light of appropriate energy, electrons and holes are generated in bulk of the semiconductor particles and travel to the surface, eventually being utilized in water reduction and oxidation reactions respectively, on catalytically active surface sites or external co-catalysts deployed on the surface. During these processes, there are several possible pathways for recombination of the electron hole pair leading to energy wastage, such as grain boundaries, lattice defects and surface sites.
Recombination can be prevented if photo generated electrons and holes are well separated from each other spatially, i.e. photogeneration and utilization sites as well as conduction pathways are physically separated in space within the structure of the semiconductor. Consequently, catalytic activity, which depends on the availability of the photogenerated electrons, can be enhanced if such a spatial separation is provided.
Another important parameter depends on the structural characteristics of valence and conduction bands, wherein holes and photogenerated electrons and holes are located respectively and in semiconductors with bulk 3D structures, they are structurally close to each other enhancing the chances of recombination. Hence, structures with inherent separation of photogenerated charges spatially, are ideal for efficient photocatalysis. Such a phenomenon is exploited by nature in utilizing solar energy whereby photogenerated charges are separated spatially by cascade processes. Solid oxide structures can be envisaged which have intrinsic structural anisotropy leading to separate sites for charge generation and electron conduction pathways, effectively separating holes and electrons.
In view of the aforesaid, a suitable photocatalytic material should possess a sufficiently small band gap for utilizing more abundant visible light region in the solar spectrum. The valence band and conduction band positions with respect to reduction and oxidation potentials of water should be appropriate to drive overall water splitting.
Lately, a lot of attention has been garnered by layered semiconductor oxides, like K4Nb6O17, members of Ruddlesden-Popper series of perovskites, layered perovskites, Sr2Ta2O7 and Sr2Nb2O7 as catalysts for H2 generation. Typically, these structures consist of sheets of transition metal oxides separated by alkali or alkaline earth metal ions, giving rise to anisotropy to a certain extent restricting movement of charges through interlayer spaces. However, in these layered compounds, the attempt is to introduce catalytic sites within the interlayer spaces thereby achieving partial space separation of the charges or spatially separate H2 and O2 evolution sites reducing the backward reaction. Moreover, high band gap energies of these compounds limit their usage to only the UV light region.
Layered structures with well-defined conduction pathways separated from photo generation sites can be envisaged to address the problems posed by layered oxides effectively. InMO3(ZnO)m are a series of oxides form one such family of compounds which are conventionally studied for their excellent thermoelectric properties as well as transparent conducting oxides. The enhanced conductivity is suggested to be due to a spatial separation of the carrier donors located in insulating layers and the conducting layers which transfer the carriers effectively. Spatial separation in InMO3(ZnO)m is found to be much higher compared to contemporary semiconductors. This is manifested in the anisotropic nature of the electrical conductivity. Measurements on thin film and single crystals reveal higher conductivity along a-b plane. Kawazoe and co-workers (Un'no, N. Hikuma, T. Omata, N. Ueda, T. Hashimoto, and H. Kawazoe, Jpn. J. Appl. Phys., Part 2 32, L1260 (1993), T. Omata, N. Ueda, K. Ueda, and H. Kawazoe, Appl. Phys. Lett. 64, 1077 (1994), K. Yanagawa, Y. Ohki, T. Omata, H. Hosono, N. Ueda, and H. Kawazoe, Appl. Phys. Lett. 64, 2071 (1994)) suggested that layers formed by edge sharing MO6 octahedra, where M is a p-block metal ion, may act as electron conducting pathways facilitating electrical conductivity. The unique electronic and band structure resulting from such a structural anisotropy makes this series, potential materials for addressing recombination issues associated with semiconductor photocatalysts.
In this context, there remains a need in the art for a simple and economical photocatalytic water splitting process catalyzed by structurally anisotropic compounds with photo-generation sites and electron conduction pathways which are spatially separated structurally.
Therefore, it will be of advantage to explore photo catalysts that have the above mentioned properties and structures and provide them as efficient photo catalysts for systems to evolve H2 by water splitting. But such catalysts should preferably satisfy the need to maintain the costs of the process of H2 generation. Rather it would be pertinent to state here that the catalyst should not be the reason for the process to not satisfy the need for an economic alternative. It would be further advantageous to provide a catalyst that functions well at the visible and the UV range.